Effects of colouration mechanism and stability of CoAl2O4 ceramic pigments sintered on substrates

Abstract CoAl 2 O 4 ceramic pigments produced by common methods have low chroma values, which limit their application in ceramic glazing, enamel glazing, and ceramic product printing. The colour stability of the ceramic pigments and changes upon high-temperature sintering are important factors, especially in 3D printing. In this study, CoAl 2 O 4 ceramic pigments were synthesized using a co-precipitation method and both the colouration mechanism and stability of CoAl 2 O 4 ceramic pigments were investigated. The Co content, Co ion valence, and coordination number were found to be important factors affecting the CoAl 2 O 4 ceramic pigments. When the Co content was excessively high, either Co 3+ was produced or Co 2+ entered the octahedron spaces of the spinel structure. The coordination field of the Co ions was consequently altered and the colour transitioned from blue to green. After sintering on substrates at 1300 °C, the brightness and chroma of obtained CoAl 2 O 4 coatings were superior to those of the pigment powders prior to sintering, and the colour of CoAl 2 O 4 with a relatively high Co/Al ratio changed from green to blue. The latter pigment may have formed a partially inverse spinel structure upon sintering at a high temperature. The probability of Co 2+ entering the octahedron spaces increased; concomitantly, the number of Co 2+ in the tetrahedron spaces decreased, resulting in an increase in the coordination number of Co. In addition, Co 3+ in the ceramic pigments was transformed into Co 2+ by deoxidation reactions during the coating sintering process, thereby increasing the chroma of blue CoAl 2 O 4 pigments.